Composite materials respectively composed of reinforcing fibers and a matrix resin are widely used as sporting goods such as golf shafts, fishing rods and tennis rackets, and for aerospace and general industrial applications, since they are light in weight and excellent in mechanical properties.
Fiber reinforced composite materials are produced by various methods. It is widely practiced to use a sheet obtained by impregnating reinforcing fibers with a matrix resin as an intermediate product called a prepreg. In this method, plural prepreg sheets are laminated and heated to obtain a molded product.
The matrix resin used for the prepreg can be either a thermosetting resin or a thermoplastic resin, but in most cases, a thermosetting resin is used. Above all, an epoxy resin is mainly used.
When prepreg sheets obtained by using an epoxy resin are used, the tackiness between the prepreg sheets and the drapability of the prepreg often come into question. These properties greatly affect the working efficiency in the handling of the prepreg.
If the tackiness of the prepreg is too small, the prepreg sheets overlapped and pressed in the prepreg sheet lamination step are soon separated to inconvenience the lamination work. In this case, the working environment temperature must be raised to a level at which moderate tackiness can be obtained.
On the contrary, if the tackiness of the prepreg is too large, prepreg sheets which happen to be overlapped by any error stick to each other due to their own weight, and it is difficult to separate them for correction.
Furthermore, if the drapability of the prepreg sheet is poor, the prepreg sheets are so hard as to remarkably lower the lamination workability, and the laminated prepreg sheets cannot be accurately adapted to the curved surface of the mold or the form of the mandrel, to be creased or to get the reinforcing fibers broken, for forming defects in the molded product. Also in this case, the working environment temperature must be kept high, and it is difficult to keep the balance between drapability and tackiness. So, these properties are very large problems for molding work.
These problems are especially liable to be caused when a golf shaft or fishing rod, etc. is molded by winding the prepreg sheets around the mandrel. If the balance between tackiness and drapability is improper, the prepreg sheets wound around the mandrel are exfoliated in a short time, to make molding difficult.
Furthermore, if the prepreg sheets are allowed to stand before use, their tackiness declines rapidly in several hours, and they can be only poorly wound around the mandrel, to make lamination work difficult. So, it is also required to keep moderate tackiness for a long time.
The tackiness and drapability of the prepreg are mainly dominated by the viscoelasticity of the matrix resin. In general, the viscoelasticity of an epoxy resin greatly depends on the temperature, and if the working environment temperature varies seasonally, the tackiness and drapability vary, and sometimes working cannot be effected.
Furthermore in the molding with a prepreg used, resin flow often comes into question. The resin flow refers to the flow of the resin when the temperature rises in the molding step. In general, since the molding with a prepreg used is effected under pressure, large resin flow causes the resin to flow out, and the product tends to deviate from design values of resin percentage and thickness. Furthermore, if the resin flow is large, a phenomenon that fine bubbles in the resin gather during molding, to form voids in the molded product, for lowering the strength of the molded product.
In recent years, golf shafts, fishing rods, etc. are reduced in weight, and prepregs suitable for light weight design are desired. Prepregs using fibers high in elastic modulus, especially carbon fibers high in elastic modulus as reinforcing fibers are especially demanded in the market in recent years since they allow easy light weight design.
However, the use of carbon fibers high in elastic modulus as reinforcing fibers lowers the drapability of the prepreg, and therefore, there is also a tendency to show the nature that the resin on the surface of the prepreg settles with the lapse of time, to lower tackiness. Therefore, if any conventional resin is used, both tackiness and drapability are insufficient disadvantageously.
For higher drapability, lowering the viscosity of the resin is generally considered to be effective. However, simply lowering the viscosity of the resin causes the disadvantages based on the large resin flow as described above.
Epoxy resin compositions containing a high molecular epoxy resin for the purpose of optimizing the tackiness and drapability of the prepreg to improve the molding workability are disclosed in Japanese Patent Laid-Open Nos. 62-127317 and 63-308026. Furthermore, Japanese Patent Laid-Open No. 2-20546 discloses an epoxy resin composition containing a nitrile rubber modified epoxy resin for the purpose of optimizing drapability and resin flow.
However, even though these methods could improve either the tackiness or drapability of the prepreg, they have a disadvantage that the balance between both the properties is improper or that the balance is achieved at the sacrifice of the mechanical properties of the molded product obtained.
Furthermore, as a method for improving the tackiness, etc. of the prepreg, it is known to add a high polymer such as a thermoplastic resin or elastomer to an epoxy resin. For example, adding a polyvinyl formal resin as disclosed in Japanese Patent Laid-Open Nos. 58-8724 and 62-169829, adding a polyvinyl acetal resin as disclosed in Japanese Patent Laid-Open Nos. 55-27342, 55-108443 and 56-2119, adding a polyvinyl butyral resin as disclosed in Japanese Patent Laid-Open No. 52-30187, adding a polyester polyurethane as disclosed in Japanese Patent Laid-Open No. 5-117423, adding a polyvinyl ether as disclosed in Japanese Patent Laid-Open No. 4-130156, etc. are known.
However, adding such a high polymer raises the viscosity of the resin, for lowering drapability, and it is difficult to find any resin satisfactory in both drapability and tackiness especially in any prepreg using carbon fibers high in elastic modulus.
Japanese Patent Laid-Open Nos. 2-92920 and 4-46923 disclose compositions consisting of an epoxy resin, polyester based thermoplastic elastomer and curing agent. However, these compositions are intended to improve the tensile strength and impact resistance of composite materials, and nothing is discussed about the problem of improving tackiness at the sacrifice of drapability and impregnability, while no means for solving the problem is suggested.
European Patent No. 381625 (corresponding to Japanese Patent Laid-Open No. 2-233754) discloses an epoxy resin composition comprising epoxy resin, liquid copolymer based on butadiene and acrylonitrile, and segmented copolymer such as copolyester, copolyamide, etc., which is used as a hot melt adhesive for structures, matrix resin or surface coating material. However, for this composition, the problem of improving tackiness at the sacrifice of drapability and impregnability is not discussed at all, and no means for solving it is suggested. In addition, if a liquid copolymer based on butadiene and acrylonitrile is added, the resin composition obtained and cured is low in heat resistance and elastic modulus, and the fiber reinforced composite material obtained by curing the prepreg produced by using the resin composition as the matrix resin is not sufficient in such physical properties as heat resistance and 0.degree. compressive strength.
Furthermore, the fiber reinforced composite material obtained by curing the prepreg produced by simply using the epoxy resin composition containing such a thermoplastic elastomer as the matrix resin is insufficient in such physical properties as inter-layer shear strength, 90.degree. tensile strength and no compressive strength. This phenomenon is surmised to be caused by the low adhesiveness between the matrix resin and the reinforcing fibers.
On the other hand, many attempts were made to improve the inter-layer toughness for enhancing the impact resistance, especially compressive strength after impact of any fiber reinforced composite material obtained by laminating and curing prepreg sheets. For example, U.S. Pat. No. 3,472,730, U.S. Pat. No. 4,539,253 (corresponding to Japanese Patent Laid-Open No. 60-231738), U.S. Pat. No. 4,604,319, Japanese Patent Laid-Open Nos. 51-58484 (Japanese Publication No. 58-31296), 54-3879, 56-115216, 6044334, 60-63229, 63-162732, and 58-205758, and Japanese Patent Publication No. 61-29265 disclose techniques for enhancing the inter-layer strength by arranging a film or fine particles, etc. of a thermosetting resin or thermoplastic resin.
However, unlike the case of using, as the matrix resin, a composition obtained by dissolving a thermoplastic resin into an epoxy resin, the thermoplastic resin and the epoxy resin composition exist in quite discrete phases. If a thermoplastic resin film covers the entire surface of the epoxy resin composition, the surface loses the tackiness of the epoxy resin composition. If thermoplastic resin particles are used, the epoxy resin composition is sufficiently exposed on the surface, and does not lose its tackiness, but the tackiness almost remains at the level achieved without using the grains, and cannot be further improved. Moreover, no material satisfactory in heat resistance, tackiness, impact resistance and the strength as the entire composite material could be obtained.
Furthermore, since golf shafts, fishing rods, etc. are reduced in weight, there is a higher demand for a prepreg higher in reinforcing fiber content.
In general, if the reinforcing fiber content is made higher, the amount of the resin distributed on the surface of the prepreg is decreased to lower tackiness. If a resin high in viscosity containing a high polymer as described before is used for example to improve tackiness, the high viscosity makes impregnation difficult in addition to the difficulty of impregnating the reinforcing fiber bundles with a resin at a high reinforcing fiber content, to lower the quality of the prepreg such as smoothness.
Especially in the case of a prepreg sheet using carbon fibers high in elastic modulus as reinforcing fibers and high in reinforcing fiber content, it is very difficult to satisfy all of tackiness, drapability and quality.
The object of the present invention is to provide a prepreg excellent in tackiness, drapability and quality and also excellent in physical properties after curing, even if carbon fibers high in elastic modulus are used or even the reinforcing fiber content is high.